Internally phosphine-stabilized zirconocene cations employing substituted ((diarylphosphino)methyl) cyclopentadienyl ligand systems

The dimethylzirconocene complex [(Cp-CMe2-PAr2)(2)ZrMe2], 2a (Ar = p-tolyl), was treated with 1 molar equiv of B(C6F5)(3) to yield the salt [(Cp-CMe2-PAr2)(2)ZrMe+MeB(C6F5)(3)(-)], 3a. The X-ray crystal structure analysis of 3a shows that both -PAr2 units are intramolecularly coordinated to zirconium in a close to C-2-symmetric arrangement with the [Zr]-CH3 group placed in the central position in the bent metallocene sigma-ligand plane. Treatment of 2a with 2 equiv of B(C6F5)(3) generates the highly reactive dication system [(Cp-CMe2-PAr2)(2)Zr2+] (4 with two MeB(C6F5)(3-) anions). The highly electrophilic cation 4 abstracts chloride from, e.g., dichloromethane solvent to yield [(Cp-CMe2-PAr2)(2)Zr-Cl+] (5, with MeB(C6F5)(3)(-) anion). The same cation (5', with ClB(C6F5)(3)(-) anion) was obtained from the reaction of [(Cp-CMe2PAr2)(2)ZrCl2] (1a) with B(C6F5)(3). The C-2-symmetric, internally -PAr2-stabilized dication adds acetonitrile or 2,6-dimethylphenyl isocyanide to give the respective C-2-symmetric donor-ligand adducts in which the -PAr2 coordination to zirconium is retained. The complexes 3a (Delta G double dagger(enant) (300 K) = 14.0 +/- 0.5 kcal/mol) and 5 (Delta G double dagger(enant) (360 K) = 17.5 +/- 0.5 kcal/mol) show dynamic NMR spectra due to an intramolecular enantiomerization process proceeding with a rate-determining cleavage of the Zr-P linkages. Complex 5 was also characterized by an X-ray crystal structure analysis.